This invention relates to semi-solid aluminum alloys, and more particularly, it relates to a method of casting and thermally transforming bodies of aluminum alloys from a dendritic structure to a non-dendritic structure and forming the thermally transformed bodies.
Most aluminum alloys solidify to form a dendritic microstructure that is not well suited to most metal forming operations. In addition, the dendritic microstructure is not well suited to forming in the semi-solid state. However, it is well known that microstructures obtained when the alloy is heated and transformed to a globular or spherical phase are more susceptible to forming in the semi-solid state. That is, when the body is heated, a transformation is obtained from the dendritic microstructure to a globular or spherical phase contained in a lower melting eutectic matrix. After rapid cooling, the alloy retains the globular or spheroidal phase. If the body is reheated to between liquidus and solidus temperature, the transformed phase is retained. Thus, the alloy is provided in a thixotropic state which provides for ease of forming because the metal can be forced into a mold utilizing smaller forces than normally required for the solidified form. Another advantage of using semi-solid metal for forming is a decrease in shrinkage of the formed part on solidification.
However, transforming the alloys from the dendritic microstructure to spheroidal or globular phase retained in the lower melting eutectic matrix is not without problems. For example, U.S. Pat. No. 5,009,844 discloses a semi-solid metal-forming of hypoeutectic aluminum-silicon alloys without formation of elemental silicon. The process comprises heating a solid billet of the alloy to a temperature between the liquidus temperature and the solidus temperature at a rate not greater than 30.degree. C. per minute, preferably not greater than 20.degree. C. per minute, to form a semi-solid body of the alloy while inhibiting the formation of free silicon particles therein. The semi-solid body comprises a primary spheroidal phase dispersed in a eutectic-derived liquid phase and is conducive to forming at low pressure. According to the patent, a billet having a quiescently cast microstructure characterized by primary dendrite particles in a eutectic matrix is heated at the slow rate and maintained at the intermediate temperature for a time sufficient to transform the dendrite phase into the desired spheroidal phase. However, slow heat-up rates can lead to microporosity caused by hydrogen adsorption. This results in inferior properties. According to this patent, rapid heat-up rates of hypoeutectic aluminum-silicon alloys to the semi-solid condition are detrimental and produce the free silicon particles.
U.S. Pat. No. 4,106,956 discloses a process for facilitating extrusion or rolling of a solidified dendritic aluminum base alloy billet, or the like, by heating the billet to provide an inner liquid phase of below 25%, by weight, wherein the dendritic phase has started to develop into a primary solid globular phase without disturbing the solidified character of the billet, followed by working of the treated billet. The process enables a reduction in working pressure and results in improved mechanical properties of the product. Optionally, in the case of precipitation hardening aluminum base alloys, quenching of the workpiece is effected as it exits from the die or mill, followed by artificial or natural aging. In another embodiment, the composition of the alloy of the billet being treated contains an amount of hardening constituent whereby the composition of the globular solid phase of the product approximates the composition of the alloy per se.
U.S. Pat. No. 4,415,374 discloses that a fine grained metal composition is obtained that is suitable for forming in a partially solid, partially liquid condition. The composition is prepared by producing a solid metal composition having an essentially directional grain structure and heating the directional grain composition to a temperature above the solidus and below the liquidus to produce a partially solid, partially liquid mixture containing at least 0.05 volume fraction liquid. The composition, prior to heating, has a strain level introduced such that upon heating, the mixture comprises uniform discrete spheroidal particles contained within a lower melting matrix. The heated alloy is then solidified while in a partially solid, partially liquid condition, the solidified composition having a uniform, fine grained microstructure.
U.S. Pat. No. 3,988,180 discloses a method of heat treatment which is applied to forged aluminum alloys, whereby the mechanical characteristics and resistance against corrosion under tension are increased considerably. The method is characterized by heating prior to tempering, above the temperature of eutectic melting, while remaining below the temperature of the start of the melting at equilibrium. The liquid phase formed temporarily is resorbed progressively, while the formation of pores is avoided by a sufficiently low hydrogen content of the metal. The application of this procedure to several aluminum alloys made it possible to observe increases of the limit of elasticity and of the break load of the order of 7% and a non-rupture stress under tension in 30 days at least equal to 30 hb.
U.S. Pat. No. 5,186,236 discloses a process for producing a liquid-solid metal alloy for processing a material in the thixotropic state. In the process, an alloy melt having a solidified portion of primary crystals is maintained at a temperature between solidus and liquidus temperature of the alloy. The primary crystals are molded to give individual degenerated dendrites or cast grains of essentially globular shape and hence impart thixotropic properties to the liquid-solid metal alloy phase by the production of mechanical vibrations in the frequency range between 10 and 100 kHz in this liquid-solid metal alloy phase.
European Patent No. 0554808 A1 discloses the use of high levels of grain refiner to produce billets which need fine globular microstructure to show the necessary thixotropic behavior. The process discloses the manufacture of shaped parts from metal alloys consisting of bringing metal alloys to a molten state and using a conventional casting process to produce a simple geometric form. Then, by heating up to a temperature between the solidus and liquidus lines, a solid-liquid mixture is produced, this mixture having a melt matrix with distributed, founded, primary particles exhibiting thixotropic properties, and after a holding time, the material is conveyed to a shaping plant. In this process, to metal alloys in a liquid state is added an unexpectedly high amount of known grain refiner. After adding the unexpectedly high amount of grain refiner, the melted metal can be cooled to any desired temperature below the liquidus line and thereafter heated to a temperature between the solidus and the liquidus and held there for a time from a few to 15 minutes.
For AA (Aluminum Association) Alloy 356 (AlSi7Mg), it was disclosed that for titanium or titanium and boron grain refiner contents less than 0.18% Ti, the primary phase consisted predominantly of large dendrites, even when the sample was held for 1 hour at 578.degree. C. Only for higher amounts of grain refiner, e.g., 0.25% titanium, it was revealed that there were isolated rounded primary particles within a holding time of 5 minutes. The same results were obtained even if the temperature was first raised to 589.degree. C. Also, the patent disclosed that at conventional grain refiner levels, the liquid eutectic drained from the sample. The grain refiner is added to produce a smaller grain size that increases the rate for converting to the rounded grains. However, adding high levels of grain refiner can adversely affect the properties of the product and adds greatly to its cost. Further, when long holding times are involved, this often results in high porosity and excessive coarsening of globular grains. High levels of TiB.sub.2 grain refiner can result in machining problems. That is, the TiB.sub.2 particles can result in excessive tool wear.
French Patent 2,266,749 discloses producing a metal alloy consisting of a mixture of liquid and solid phases in a proportion which allows the said alloy to transitorily behave like a liquid when under the influence of an exterior force, at the moment when it is shaped into a mold, and then instantaneously recover its solid properties when the force ceases. According to the patent, this procedure consists of producing the said alloy at a temperature between the equilibrium solidus and liquidus temperatures, chosen so that the preponderant fraction of liquid phase is at least 40%, and preferably in the region of 60%, and maintaining this said temperature for a time between a few minutes and some hours and preferably between 5 and 60 minutes, in a manner so that the primary dendritic structure has begun to evolve towards a globular form.
PCT Patent WO 92/13662 (Collot) discloses producing a fine grained aluminum alloy ingot by solidification under high pressure to avoid porosity. The ingot is then reheated into a semi-solid state and pressed into a mold under pressure to produce shaped pieces which have a fine globular structure free from porosity.
In another approach to preventing or destroying the dendritic microstructure, the metal, while in the liquid-solid state, is stilted or agitated to destroy or prevent the dendritic structure from forming. Such processes are disclosed, for example, in U.S. Pat. Nos. 4,865,808; 3,948,650; 4,771,818; 4,694,882; 4,524,820 and 4,108,643.
It should be understood that upon heating a body, e.g., billet or other shaped aluminum alloy product, to a temperature between liquidus and solidus, the solid shape or appearance of the body is normally not changed significantly and yet the primary phase or dendritic microstructure changes or transforms to a globular or spheroidal form with the size of the globular or spheroidal form dependent on the size of the dendritic structure and grain size at the start. Further, it should be noted that this transformation from dendrite form to globular phase takes place while the grains remain generally in solid form. However, the globular form is contained in a lower melting eutectic alloy matrix which matrix becomes molten. Generally, the molten portion of the aluminum body does not exceed about 30 to 40% by weight. However, the outward appearance of the aluminum body is not substantially changed from that of a solid body. Yet, the body takes on the attributes of a plastic body and can be formed by extruding,. forging, casting, rolling, stamping, etc., with greatly reduced force.
In spite of these teachings, there is still a great need for a process that permits economic transformation of a cast product such as aluminum ingot, billet, slab or sheet to a spheroidal or globular phase for ease of semi-solid forming or forming into products without altering the chemistry of the alloy.